1. Field of the Invention
The present invention relates to a semiconductor switch control device, a semiconductor switch device, a load drive system, and a semiconductor switch control program. More specifically, the present invention relates to a semiconductor switch control device to protect a semiconductor switch from breakdown due to overload.
2. Description of Related Art
In recent years, computerization has been advancing in order to realize safety, comfort, and energy saving of automobiles. As an example, a mechanical relay has been replaced with a semiconductor switch, which contributes to downsizing of a device mounted on an automobile. Further downsizing has been required in the device, and in accordance therewith, miniaturization of a semiconductor chip design rule of a semiconductor switch has been advancing. More specifically, the size of a semiconductor chip has been gradually miniaturized from 1.2 μm, 1.0 μm, 0.5 μm, 0.35 μm, to 0.25 μm, and further miniaturization will be desired from now on.
Although the miniaturization of the design rule increases a density of a current that can be flowed in the semiconductor chip per unit area, a new problem of heating has been raised due to the increased current density. Especially, decrease of a breakdown tolerance due to heating in a transient state, mainly due to the heating in a load short-circuit, is a major problem. Electronic manufacturers and semiconductor manufacturers conduct research of a temperature detecting means, a current limiting means, a current detecting means and the like in order to protect semiconductor switches.
For example, a power supply control device is disclosed in a patent document 1 (Japanese Unexamined Patent Application Publication No. 2000-299631).
In FIG. 9, an ON resistance of a semiconductor switch 124 is employed as a load current detecting means. Then, a temperature of the semiconductor switch 124 is detected by temperature detecting means (D1 to D4, and a differential amplifier 105), a load current is detected by current detecting means (an ON resistance of the semiconductor switch 124 and a differential amplifier 106), and on the basis of the temperature detected result, the level of the reference potential of a comparator for judging an excess current is shifted by a driving control unit 107. When this temperature corrected reference potential is exceeded, an FET 102 is controlled to OFF. And in this way, an excess current braking function in the control of power supply from a power source 101 to a load 103 is provided.
According to this configuration, (1) heat loss of the power supply control device can be suppressed without using additional current sensing resistor (so-called shunt resistor), (2) the problem of the temperature dependency of the ON resistance of the semiconductor switch can be overcome by compensating the temperature upon detection of the load current and then the high-accuracy detection of the load current is realized, and (3) individual control for each load is enabled.
Further, a patent document 2 (Japanese Unexamined Utility Model Application Publication No. H1-37135) discloses a protection device of a power transistor.
In the patent document 2, each of a current that flows in a power transistor and a voltage that is applied is detected. Then, a product of the detected current and the voltage is calculated. Predicting an increase of the temperature when the value of the product of the current and the voltage is kept for a certain period of time, the power transistor is turned off after a certain delay time in accordance with the value of the product of the current and the voltage. By varying the time for the off control in accordance with the heating value, it is possible not to operate a protective function for the instantaneous excess current that does not cause the thermal destruction of the switch and to execute off control in accordance with time when increase of the temperature of the power transistor would cause destruction.
The off control by the current detection as disclosed in the patent document 1 appears effective. But we have now discovered that it is rarely used in practice due to its insufficiency of the response of a lamp-driven rush current as below. More specifically, when turning on the lamp as a load, the rush current that is six to ten times larger than a nominal rating current of the lamp. This rush current considerably varies depending on the temperature of the lamp itself, the voltage that is applied, and the lamp manufacturer and so on.
Further, a lamp that is not recommended by any manufacturer may be employed as a load by a user of an automobile. When the setting of the reference current Iref was improper, the rush current would exceed Iref in a lamp-driven lighting step, which makes it impossible to light the lamp due to the off control despite its normal state.
Furthermore, if the value of Iref is set to high in order to avoid such a situation, the semiconductor switch whose rated current is high is required to be used, which may increase the cost.
The patent document 2 solves the problem of the patent document 1 by considering the time elapse with the heating value, and turning off the semiconductor switch only when it is expected that the temperature of the semiconductor switch causes the thermal destruction. However we have now discovered that according to the patent document 2, the control is performed based on the heating value which is derived from the power consumption and its duration time. Any problem may be raised, for example, the semiconductor switch is destructed earlier than expected when the ambient temperature is high. On the other hand, when the ambient temperature is low, the semiconductor switch is frequently turned off even when there is no need to perform off control, which makes it impossible to realize stable operation.